Electronic amplifier



June 30, 1942. D H, NL N ETAL 2,287,942

ELECTRONIC AMPLIFIER Filed Dec. 7, 1938 AMPLIFIER 34 AND RECTIFIER 20 lVOLTS /l\ AND 1 EIXIOG O l I \sl -4- 52 E5, I j nk .25 .50 .75VINVENTORS [00 SECONDS D. H. NELSON BY W. DOQUCKINGHAM ATTORNEY PatentedJune so, 1942 ELECTRONIC AMPLIFIER.

Dale H. Nelson, Water Mill, and William D. Buckingham, Southampton, N.Y., assignors to The Western Union Telegraph Company, New York, N. Y., acorporation of New York Application December 7, 1938, Serial No. 244,330

8 Claims.

This invention relates to electronic amplifiers and particularly to anamplifier for amplifying low frequency signals.

In many instances an amplifier is desired for amplifying weak signals soas to enable such signals to operate alarms, meters, recorders and thelike. Generally an amplifier of this character is designed to provideuniform amplification of signals of varying amplitude and thus'is notsuited to the amplification of signals which vary over a wide range, forexample, where the stronger signals are of the order of one hundred orone thousand times as large as the weaker signals. In accordance withthe invention an amplifier suitable for amplifying signals of widelyvarying strength is provided by incorporating anexponential control ofthe gain of the amplifier whereby all signals are amplified to anextentvarying in accordance with the strength of the signals and theamplified signals may be interpreted in terms of the knowncharacteristics of the exponential gain control system.

In general terms, the primary object of the invention is to provide animproved amplifier having exponential output characteristics.

Another object of the invention is to provide an amplifier of theabove-stated character which is particularly adapted for amplifying .lowfrequency signals of the order of one or two cycles a second or less.

Another object of the invention is to provide .means whereby minuteelectrical signals may be indicated or recorded accurately for purposesof comparison or measurement, particularly in cable surveying apparatusor the like where the signals vary in amplitude over a wide range. Itwill be apparent that if one signal is only about one percent of theamplitude of another signal, the usual indicating or recording devicesdo not permit of an accurate comparison of the two signals where theinstrument is provided with a uniform scale, since the smaller signalcould not be detected or the larger signal would be oif the scale. If,however, the smaller or weaker signals are amplified to a greater extentthan the larger or stronger signals according to a known exponentiallaw, the deflections or recorded amplitudes of the signals may -beaccurately measured or compared. It is therefore a still further objectof the invention to provide a novel amplifier of i this character whichmay be used with standard indicating or recording meters. V

Other objects and advantages of the invention will appear from thefollowing description of the electrical amplifying system shown in theaccompanying drawing, wherein:

Fig. 1 is a diagrammatic view of a system em ratus such as thatdisclosed and claimed in our prior Patent No. 2,238,072, granted April15, 1941, for Method of and means for locating concealed bodies. Inapparatus of this character the signaling impulsesgenerated in thepick-up coils in the sled, as it is dragged over the cable, areexceedingly weak and vary over a wide range and it is necessary tomeasure or record such signals for comparison with extreme accuracy forproper interpretation.

In accordance with the present invention the signals from the pick-upcoils mounted in the sled aream'plified by a special amplifier in whichthe output voltage varies as some root of the input voltage, forexample, the square root of the input voltage, the output signals beingused to operate a recording meter whereby accurate comparison of the twoimpulses produced as the sled passes over the cable is facilitated. Inaccordance with the further feature of the invention, the amplifiercharacteristics may be readily varied to obtain any desired exponentialcharacteristics. Obviously, the invention is not limited to theparticular application mentioned but may be applied to the measurementor determination of minute electrical signals from vari- 0118 sources.

Since the signals generated in the coils H areoflow frequency and inreality of the character of direct current impulses, a chopper isemployed asexplained in our prior application for the purpose ofconverting signals into alternating current or oscillations of anysuitable frequency -that will be efiicientlyamplified by the electronicamplifier. Where the signals are of a sufficient high frequency or inapplications where an amplifier is available to handle the initialsignal frequency, the chopper or interrupter 13 may be omitted.

The amplifier comprises two push-pull transformer coupled stagescomprising the space-discharge devices or vacuum tubes it and i1connected to the secondary winding of the transformer H3 and thespace-discharge devices or vacuum tubes and is connected to thesecondary winding of the intermediate coupling transformer 20. Thevacuum tubes i6, ll, l8 and I9 are preferable voltage amplifier pentodetubes of the 6K? type providing stable high-gain amplification, but theinvention is not limited to this type of tube. In general, vacuum tubesmay be employed in the amplifier which are of the type of which the gainmay be readily controlled, as for example any variable-mu tube which canbe controlled by varying the grid bias to varyv the overall gain.

In the preferred arrangement shown, the con-- trol electrodes of thetubes I6 and I! are connected to the terminals of the secondary windingof the transformer i4 and the grid return circuit including conductor 2|connected to the midpoint of said secondary winding is connected to asource of variable potential dependent upon the amplitude of the outputvoltage for controlling the gain of the first stage of the amplifier. Aswill be explained, the application of the gain control voltage isaccomplished in a particular manner, i. e., the variation of the gain ofthe amplifier is rapid enough to control the individual signal impulsebut not so fast as .to cause motor boating (the generation of lowfrequency oscillations). In general terms, this essential result isaccomplished by controlling the rate of change of the gain controlpotential derived from the output of the amplifier and applied to one ormore stages thereof. The gain of the second amplifier stage is similarlycontrolled by a bias potential applied through conductor 22 to controlelectrodes of the vacuum tubes [8 and I9.

The final stage of the amplifier may be coupled through a transformer 25to the output circuit 26. A resistor 21, for example of 0.5 megohm, isbridged across the circuit 23 and the alternating output voltage isrectified by a rectifier '28 in case a meter such as the meter of thedirect current recording type is utilized to provide an indication ofthe magnitude of the signals.

The output circuit of the amplifier is also connected by a conductor 32toan'amplifier and rectifier 33 of conventional construction arranged togenerate a direct current in the'circuit thereof proportional to theamplitude of the output voltage of the amplifier. The circuit 34 isbridged by a condenser35, for example of 1 mf. capacity, and by apotentiometer 36 having, for example, a total resistance of 50.000 ohms.The variable tap 31 of the potentiometer 36 is connected to theconductors 2| and 22 for controlling the gain in the respective stagesof the amplifier through the filter circuits shown. when the variabletap of the potentiometer 38 is adjusted to the point indicated by thedotted line 33, no exponential control is exercised upon thecharacteristics of the amplifier, which then operates as a linearamplifier, a battery 33 furnishing normal bias forthe control electrodecircuits of the vacuum tubes l6, l1, II and [9.

In accordance with one featiire of the invention, when the variable tap31 is moved from the zero position the gain of the amplifier varies inaccordance with the nth root of the input signal. Thus the tap may bemoved to such a position that the gain of the amplifier is'prgf--portional to the square root of the input signal and if the tap is movedfurther, the gain will be proportional to the cube root of the inputsignal, etc. Ordinarily, the variable tap of the potentiometer 36 issuitably adjusted to provide the desired exponential characteristic ofthe amplifier and then is left at such adjustment during the subsequentuse of the apparatus.

The rectified current proportional to the output signal of the amplifieris translated into a bias potential by the drop across the potentiometer36 and is thus added to the constant bias potential of the battery 39.The combined bias potential varying in accordance with the instantaneousoutput voltage is applied tothe conductors 2| and 22 arranged to controlthe gain of the respective stages of the amplifier as pointed out above.The gain control circuit including conductor 2| contains a filter systemincluding an inductance 40, for example, of 50 henries, a seriesresistor 4| and grounded condensers 42 and 43 connected to the terminalsof said resistor. The resistor 4| may be of the'order of 50,000 ohms.-The capacities of the condensers 42 and 43 are critical depending uponthe durationpr frequency of the input signals since the exponential gaincontrol must follow each individual signal so that the gain of theamplifier is a maximum when the input signal is small and isreduced to aminimum upon the occurrence of a larger input signal of maximumamplitude. In the case of signals picked up from a cable depthometerarrangement as described above where the signals are not less thanmilliseconds in length, the condenser 42 is preferably of 0.5 mi. andthe condenser 43 of 0.1 mf. capacity. In

this case the filter will not pass an alternating current of greaterfrequency than ten cycles per second, the time constant of the gaincontrol being also affected to a certain extent by the condenser 35.

A somewhat similar filter circuit is provided in series with theconductor 22 in the grid return circuit of the second stage of theamplifier, said filter circuit including an inductance 45, for exampleof 50 henries, a series resistor 46 of 100,000

ohms and a shunt condenser 41 of 0.1 mf. capacity. However, theseconstants of'theelements of the second filter circuit are not criticalsince the gain control is primarily effected through the first stage ofthe amplifier.

The interconnections of the tubes l6, I1, [8 and I 3 .of the amplifierand the other detailed circuits of the cathode, anode and the control,screen and suppressor grids of these tubes are in accordance withgeneral principles well known in the art except as pointed out below.When the input signal is impressed upon the control grids of the tubes[6 and I! through the transformer l4, the signal is amplified by thetubes 16 and I1 and impressed upon tubes l8 and I! through theinterstage coupling transformer 20. In the tubes l8 and I! the gridreturn circuit is connected to the upper and lower control electrodeswhich are the suppressor grids-whereas the suppressor grids of-the tubesl6 and I! are grounded. The connection of the suppressor grids to thegrid return circuit results in the exercise of a greater control uponthe amplification factor of the tubes l8 and I3 by variation of the gaincontrol potential and this connection could also be utilized in thefirst stage of the amplifier if desired.

Fig. 2 shows the characteristics of theamplifier for two successiveinput signals, one of which curves 50 and it and, as shown, each signalimpulse has a duration of approximately 400 milliseconds. The curves 52and 53' represent the output signals produced by impulses 5D and 5!respectively, as recorded by the meter 30, the impulses 52 and 53 beingreduced in scale with respect to the input signals. The curves 5i and 55represent the variation in the overall gain of the amplifier when theimpulses 5B and 5! are impressed upon its input circuit. It is assumedthat the potentiometer 35 has been adjusted to provide a variation inthe gain of the amplifier proportional to the square root of theamplitude of the input signal. Referring to Fig. 2, it will be notedthat as the" potential of the input signal Ei increasesfrom zero to amaximum, the rectified portion of the signal in the .output circuitalters the'bias of both stages of the amplifier through the circuitsincluding conductors 2| and 22, and the overall gain is reduced asindicated by curve 54 which has a minimum point corresponding to themaximum value of the input signal Ei. Thus it will be'seen that the timeconstant of the gain control is such as to enable the gain of theamplifier to follow variations in the amplitude of the individualsignals. It will be understood that the signals impressed upon anamplifier embodying the invention may .be either direct current signalsor one half of an alternating current signal or the envelope ormodulation of a signal on an alternating current. If the time constantof. the gain control is suitably designed in accordance withthe durationof the signals for signal envelopes applied to the I amplifier inaccordance with the principles set forth above, it will be apparent thatthe amplifier may be utilized in many difierent applications. v

It will be noted that the curves 52 and 53, representing the outputpotential E are more nearly of the same magnitude than the input signalEi represented by the curves and 5|. In the curves shown by way ofexample, the output signals are proportional to the square root of theinput signals- It will be apparent that with an amplifier embodying theinvention minute input signals of widely varying amplitude may beaccurately compared or measured since, after amplification, theirmagnitudes beara known predetermined relation to the amplitudes of theinitial signals but are not directly proportional which would be thecase if a linear amplifier were employed. V

The amplifier embodying the invention is widely difi'erent inarrangement andresult from the ordinary type of vacuum tube amplifierprovided with automatic volume control. In the ordinary amplifier thetime constant of the automatic volume control is relatively longcompared to the shorter signals being amplified so that the control ofthe gain of the amplifier depends upon the average signal strength.Furthermore, in such amplifier the output is not proportional to theinput signals or any fixed power or root thereof over the entire rangeof signals and therefore would be of no utility in those applicationsfor which the present amplifier may be used, such for example as in themeasurement or comparison of individual signals.

Various modifications of the system shown and described for the purposeof explaining the invention will occur to those skilled in the art andmay be made without departing from the scope of the invention as setforth in the appended claims. n

We claim:

1. In combination, means for generating low frequency signals of theorder of one or two cycles a second, an amplifier having its inputcircuit connected to said first-named means and means connected totheoutput circuit of said amplifier for varying the gain of the'amplifier in proportion to a root of the input voltage whereby thevoltage variations in the output circuit vary exponentially but to asmaller extent than the corresponding input voltages for signals ofdifferent amplitudes, said meansfor varying the gain of the amplifierincluding means whereby the output response of the amplifier to eachin-- dividual signal impulse is controlled in accord- 'ance with themagnitude thereof.

2. In combination, means for generating low frequency signals of theorder of one or two cycles a second, an amplifier having its inputcircuit connected to said' first-named means and means associated withthe output circuit of said amplifier for varying the gain of theamplifier in proportion to a root of the input .voltage whereby thevoltage variations in the output circuit vary exponentially but to asmaller extent than the corresponding input voltages for signals ofdifierent amplitudes, the timing of said means for varying the gain ofthe amplifier be-' ing sufiiciently rapid to control the amplitude ofthe output signal corresponding to each. individual signal impulseimpressed upon the input circuit.

3. Cable surveying apparatus comprising a movable sled adapted to bedragged over the cable, a plurality of pick-up coils associated withsaid sled inwhich signals are induced in the proximity of the cable bythe movement of the i the amplifier in accordance with the square rootof the amplitude of the impressed signals, and

- recording means connected to the amplifier for recording the amplitudeof the signals after being amplified by said amplifier.

4. Cable surveying apparatus comprising movable coil pickup means inwhich signals are induced in the proximity of the cable and anelectronic amplifier connected to said pickup means for amplifying thesignals, said amplifier embodying means for varying the gain of theamplifier during the period of each signal impulseand in a manner toproduce output signals proportional to a root of the ampltude of thesignals picked up by said means.

5. Cable surveying apparatus comprising a plurality of spaced pickupcoils movable in relation to the cable to produce successive impulsestherein as the coils are moved relative to the cable, an amplifier foramplifying said signals, said amplifier including (means for varying thegain thereof during the period of each signal impulse and in accordancewith a root of the amplitude of the signals impressed thereon from thepickup coils, and recording means connected to the amplifier for malnnga record corresponding to the amplitudes of. the successive signals.

' 6. Cable surveying apparatus comprising a plurality of spaced pickupcoils movable in relation to the cable to produce successive signals asthe coils are moved across the cable, means for chopping saidignals,.amplifying means for emplifying the chopped signals, saidamplifying means including means for varying the gain of the amplifierin proportion to a root of the input Voltage at a rate suflicientlyrapid to control the amplitude of the output signal corresponding toeach individual signal picked up by said pickup coils, and recordingmeans connected to said amplifier means for making a record of theamplitudes of the successivesignals.

'7. In a low frequency signaling system, an amplifier having an inputcircuit, means for generating variable amplitude low frequency signalsof the order of one or two cycles a second and impressing said signalsupon said input circuit and means connected to the output circuit ofsaid amplifier for varying the gain of the amplifier in proportion to aroot of the input voltage, the timing of said means for varying the gainof the amplifier being sufilciently rapid to control the amplitude ofthe output signals corresponding to each of the individual signalimpulses impressed upon the input circuit. l

,8. In a low frequency signaling system, means for generating variableamplitude low frequency signals, means for chopping said signals, avariable gain amplifier upon which said chopped sig-

